1. Field of the Invention
The invention relates generally to the field of seismic exploration of subsurface rock formations. More specifically, the invention relates to seismic energy sources used to illuminate the subsurface rock formations for seismic evaluation thereof.
2. Background Art
Seismic exploration of subsurface rock formations includes so called “controlled source” or “active source” exploration, wherein one or more seismic energy sources are deployed above an area of the subsurface to be evaluated. The deployment may be near the land surface for land based surveys or in the water or on the water bottom for marine surveys. The sources are energized, and energy reflected from subsurface acoustic impedance boundaries and other features (such as diffractors) is detected by a plurality of seismic sensors deployed near the source. Recordings of the detected seismic energy are analyzed to determine various properties of the subsurface formations, for example, their geologic structure, mineral composition and/or fluid content.
The important requirements of a land based seismic energy source are that it has a high source output level, the capability to perform precisely reproducible frequency sweeps which can be recovered (e.g., by cross-correlation) as short time domain signals, compactness, and good acoustic coupling to the earth. A particular problem encountered during transmitting acoustic energy into the subsurface as experienced in seismic exploration practices is relatively poor coupling of the source to the earth due to impedance mismatch between the air and the soil or ground conditions. The impedance mismatch limits the efficiency of the transmission of seismic energy into the ground within the near surface. Such contrasting earth conditions or properties from the energy generating atmosphere act as a barrier for energy to initially propagate across the air/earth interface and into the earth's subsurface. Typically only a small percentage of the generated energy coherently enters into the subsurface. In addition the signal consistency between different source actuations (from “shot to shot”) can degrade in relation to the degree of soil inhomogeneity.
There exists a need for more efficient seismic energy sources with improved coupling and improved shot to shot coupling efficiency.